The present invention relates to a method of preparing galactooligosaccharides having a function of promoting the growth of bifidobacterium, that is, galactooligosaccharides expressed by a general formula Gal-(Gal)n-Glc (where Gal represents a galactose residue, Glc represents a glucose residue, and n represents an integer between 1 and 4), the method being able to prepare superior yields of galactooligosaccharides from lactose. More particularly, the present invention relates to a method of preparing a sweetener from lactose.
In the dairy industry, a great quantity of whey is obtained as a by-product when butter, cheese, casein protein, or whey protein is prepared from cow's milk or goat's milk. Worldwide the quantity of cheese whey alone is estimated to reach a hundred million metric tons. However, only a small proportion of less than 5% of that whey has been utilized effectively, the residual whey being disposed of. This leads to critical environmental problems in the dairy industry. Therefore, the effective use of whey, and more particularly, of lactose which is about 80% of the overall solids in whey has been desired by dairy nations in terms of the practical use of unused resources and solution of the environmental problems.
Whey and lactose therefrom have been conventionally utilized so as to be added to diectary products or drugs, and are further utilized for manufacturing syrup sweeteners and galactooligosaccharides functioning to promoting the growth of bifidobacterium.
Galactooligosaccharides have been manufactured from lactose in such a manner that .beta.-galactosidase of Aspergillus is allowed to act on lactose as disclosed in Japanese Patent Publication No. 58-20266, and in another manner such that Cryptococcus yeast is utilized as in Japanese Patent Laid-Open No. 61-236790. However, these methods which utilize the .beta.-galactosyl transfer reaction performed by .beta.-galactosidase suffer from a poor yield of galactooligosaccharide, the yield of the galactooligosaccharide from lactose stopping at a poor proportion of about 30%. Problems of this type are considered to be caused by the interruption action of monosaccharides which are produced secondarily. That is, monosaccharides can be secondarily produced by the hydrolytic reaction which takes place simultaneously with the .beta.-galactosyl transfer. Glucose which is the principle ingredient of the thus produced monosaccharides serves as an acceptor of the .beta.-galactosyl transfer, causing transfer disaccharides to be produced. Therefore, this transfer action competes with the transfer reaction which produces oligosaccharides. Furthermore, the monosaccharides inhibit the activity of the .beta.-galactosidase, causing the speed of the transfer reaction to be lowered. The quantity of the production of galactooligosaccharides reaches its maximum rate when the concentration of lactose serving as the substrate of the reaction is halved or lowered with respect to the concentration at the time of supplying the lactose. However, the concentration of monosaccharide in the solution rises in accordance with the progress of the reaction, causing the interruption to become considerable. As a result, the reaction time is lengthened and other secondary reactions take place.
Furthermore, since the product obtained from the reaction contains a large quantity of non-reacted lactose, monosaccharides which have been secondarily produced due to the hydrolytic reaction, as well as transfer disaccharides generated from the thus-produced monosaccharides, the content of the desired galactooligosaccharides is not satisfactory.
Therefore, methods to improve the yield of galactooligosaccharides with respect to lactose and to raise the content of oligosaccharides in the product obtained from the reaction have been studied. As a result, a method was disclosed in Japanese Patent Laid-Open No. 62-130695, the method being characterized in that a product in which galactooligosaccharides are contained at a considerably high degree can be obtained by allowing the reaction in which the galactooligosaccharides are produced to progress while the glucose and galactose produced by a reaction when allowing Cryptococcus yeast to act on lactose are consumed by other yeasts. However, in this method in which the reaction is allowed to progress with the monosaccharides secondarily produced during the reaction being consumed by yeast, large quantities of glucose and galactose are consumed by microorganisms without being effectively used, and the intended improvement in the yield of galactooligosaccharides from lactose cannot be achieved. Also a large quantity of yeast is required and work for removing the yeast must also be conducted after completion of the reaction. In addition, as refining work for removing the metabolite of yeast from the reacted product and other work for removing waste yeast are necessary cost is also raised.
Furthermore, the above-described interruption of the transfer reaction by monosaccharides becomes excessive as in the concentration of monosaccharides rises. Therefore, the attempt made to improve the efficiency of the reaction by raising the concentration of lactose which is the raw material in the liquid to be reacted is interrupted.
For the preparation of sweetener from lactose, there is a process wherein lactose which is worthless as a sweetener since it has a poor relative degree of sweetness with respect to that of saccharose is hydrolyzed so that glucose and galactose each of which has a high degree of sweetness are prepared. The hydrolyzing of lactose is conducted by an acid hydrolysis method and an enzyme method. As the former method requires severe conditions in the reaction thereof, not only are expensive facilities necessary but further the separation and refining of the product resulting from the hydrolysis cannot be achieved easily. Therefore, an enzyme method using .beta.-galactosidase has been widely used recently. Although the enzyme method only needs moderate reacting conditions, a large quantity of enzymes must be used in order to completely conduct the hydrolysis. Furthermore, the reaction in the enzyme method must be conducted under conditions of low lactose concentrations of 4 to 10% in order to prevent the progress of the saccharide transfer reaction which is the secondary reaction. Therefore, a large reacting apparatus and syrup condensing apparatus are necessary, increasing the cost needed for the condensation, etc. Furthermore, the degree of sweetness which can be theoreticallyachieved is at most about 0.6 even if glucose and galactose of the same mole are prepared by completely hydrolyzing lactose. Furthermore, the quality of sweetness of the sweet syrup obtained is far inferior to sugar and is not very appealing.
In order to overcome this, a method with which the degree of sweetness of the syrup can be raised and the quality of sweetness can be improved was disclosed in Japanese Patent Laid-Open No. 50-70535, the method being characterized in that glucose isomerase is allowed to act on syrup obtained by hydrolyzing lactose so as to convert glucose into fructose displaying significant sweetness. However, since the conversion ratio from glucose into fructose is at most 50% and galactose of low sweetness remains as it is, the degree of sweetness of the overall syrup is at most about 0.7 to 0.8 and the quality of the sweetness is not satisfactory.
Syrup containing the same amount of galactose as glucose also raises physiological problems as a sweetener. That is, although galactose is a necessary component of the human body, the intake of large qauntities of galactose can prove fatal for a person suffering from galactosemia, those who lack a metabolic function for converting an intake of galactose into glucose. Furthermore, galactose is considered to be a cause of cataracts in infants and the elderly whose galactose metabolism is insufficient, and should thereby avoid the intake thereof.
To this end, an object of the present invention is to overcome the above-described problems arising when galactooligosaccharides are prepared from lactose by utilizing the action of .beta.-galactosidase (or microorganisms containing enzymes of this type).
Another object of the present invention is to provide a method of preparing syrup sweetener capable of overcoming the above-described problems which arise when sweetener is prepared from lactose, the obtained syrup sweetener displaying a low degree of galactose content and exhibiting both an excellent degree of sweetness and excellent quality of sweetness.